1. Technical Field of the Invention
The present invention relates to a magneto-optical element material composed of a magnetic garnet single crystal which shows a small temperature dependency of a Faraday rotation angle, capable of being formed into a film by liquid-phase epitaxial growth (hereinafter, referred to as "LPE") method, and demonstrates a significantly small wavelength dependency of a Faraday rotation angle in a specific composition region. The present invention further relates to a Faraday element formed by superimposing an A film on a B film for broadening a usable wavelength band region by making use of a difference in wavelength dependency of a Faraday rotation coefficient between both the films A and B, wherein the A film is made from a Bi (bismuth)-substitution type rare earth-iron garnet single crystal not containing Co (cobalt) and the B film is made from a rare earth-iron garnet single crystal containing Co.
More particularly, the present invention relates to a magneto-optical element material composed of a magnetic garnet single crystal expressed by a composition formula of R.sub.3-.sub.x Bi.sub.x Fe.sub.5-v-w-y Ma.sub.v Mb.sub.w Co.sub.y O.sub.12, and, in the meantime, to a Faraday element formed by superimposing an A film on a B film, wherein the A film and the B film are composed of magnetic garnet single crystals expressed by the following composition formulas R.sub.3-x Bi.sub.x Fe.sub.5-y Ma.sub.y O.sub.12 and R'.sub.3-k Bi.sub.k Fe.sub.5-l-m-n Mb.sub.1 Mc.sub.m Co.sub.n O.sub.12, respectively.
2. Description of the Related Art
In recent years, with an erbium doped fiber amplifier (EDFA) being practically used, a 1550 nm band is being mainly adopted for optical fiber communication. As a magnetic garnet single crystal used in such a wavelength band, there is known, for example, a LPE film (equivalent to Comparative Example 1 which will be described later) having a composition of Tb.sub.1.85 Bi.sub.1.15 Fe.sub.4.75 Al.sub.0.25 O.sub.12. One of criteria for evaluating characteristics of such a magnetic garnet single crystal is a Faraday rotation coefficient .theta..sub.F (deg/cm). The larger the absolute value of a Faraday rotation coefficient of a film made from a magnetic garnet single crystal, the thinner the film thickness required to obtain a necessary Faraday rotation angle can be made. This is advantageous in that the film can be easily produced. Another criterion for evaluating characteristics of a magnetic garnet single crystal is a temperature dependency of a Faraday rotation angle. The smaller the temperature dependency of a Faraday rotation angle, the less a change in Faraday rotation angle against a change in external environment temperature. In addition, the reason why the LPE film is adopted is that the LPE method is suitable for mass-production (because of a short time required for growth of the LPE film) and thereby it is low in manufacturing cost, and further it easily allows substitution of Bi which is an element capable of significantly increasing a Faraday rotation angle on the negative side.
In recent years, a wavelength multiple transmission is expected for realizing large capacity optical communication in a 1550 nm band. The wavelength multiple transmission necessitates a wide band type optical isolator. The necessary characteristic of a magnetic garnet single crystal constituting a Faraday rotator of such a wide band type optical isolator is to ensure a small wavelength dependency of a Faraday rotation coefficient. Also the magnetic garnet single crystal is desired to be formed by the LPE method.
Bi-substitution type iron garnet single crystals used in a wide wavelength band, which have variously developed in recent years, are described, for example, in J. Appl. Phys., 70(8), Oct. 15, 1991, and Japanese Patent Laid-open Nos. 4-118623, 5-88126, 5-88127, and 8-91998.
In general, a Faraday rotation angle of a magneto-optical element material is changed depending on an external environment temperature. Accordingly, even if a Faraday rotator is set such that a polarization plane is just 45.degree. rotated at room temperature, the rotation angle of the polarization plane of the Faraday rotator becomes offset from 45.degree. with a change in temperature. As a result, the degree of cancellation of light coming in the reversed direction is lowered, with a result that an isolation characteristic of an optical isolator using the Faraday rotator is deteriorated. For example, the above-described LPE film (equivalent to Comparative Example 1) having the composition of Tb.sub.1.85 Bi.sub.1.15 Fe.sub.4.75 Al.sub.0.25 O.sub.12 does not necessarily exhibit a sufficient performance because an isolation characteristic of an optical isolator using the LPE film is considerably deteriorated with a large change in external environment temperature.
The Faraday rotation angle of a magneto-optical element material is also dependent on a wavelength. Accordingly, even if a Faraday rotator is set such that a polarization plane is just 45.degree. rotated with respect to light having a specific wavelength emitted from a light source, the rotation angle of the polarization plane of the Faraday rotator becomes offset from 45.degree. with a change in wavelength of the light emitted from the light source. As a result, the degree of cancellation of light coming in the reversed direction is lowered, so that an isolation characteristic of an optical isolator using the Faraday rotator is deteriorated.
An optical isolator, used for the above-described wavelength multiple transmission expected as large capacity optical communication at a 1550 nm band, is required to exhibit a desirable isolation characteristic at a wide wavelength band having a width of for example .+-.20 nm centered at 1550 nm, that is, the wavelength band between 1530 nm and 1570 nm. From this viewpoint, the related art magneto-optical element material, for example, equivalent to the above-described Comparative Example 1 has a very larger wavelength dependency of a Faraday rotation coefficient, and therefore, it is unsuitable for such a wide band type optical isolator.
As a material suitable for an optical isolator used in a 1550 nm band, a Bi-substitution type terbium-iron garnet single crystal having a composition of Tb.sub.3-x Bi.sub.x Fe.sub.5 O.sub.12 is disclosed in the above-described document, J. Appl. Phys., 70(8), Oct. 15, 1991. Such a material having a very small absolute value of a Faraday rotation coefficient, however, is disadvantageous in that it necessitates a large thickness of about 1.5 mm to 2 mm and thereby it enlarges the size of a Faraday rotator using the material for an optical isolator. Another disadvantage is that the magnetic garnet single crystal, which is produced by a flux process, cannot ensure a uniform composition and also it is unsuitable for mass-production because of a longer time required for production. Of magnetic garnet single crystals disclosed in the above-described documents, Japanese Patent Laid-open Nos. 4-118623, 5-88126, and 5-88127, those produced by the flux process exhibit relatively large Faraday rotation coefficients; however, those produced by the LPE method exhibit small absolute values of Faraday rotation coefficients. Further, although the above-described document, Japanese Patent Laid-open No. 8-91998 discloses a technique in which the magnetic garnet material having a similar composition is produced by a solid-phase reaction process for improving a suitability to mass-production; however, the magnetic garnet material also necessitates a large thickness of 1.5 mm to 2 mm, with a result that it is disadvantageous in terms of miniaturization of a Faraday rotator using the material.